A carbonate is a diester of carbonic acid, represented by R--O--CO--O--R' (wherein R and R' are each an alkyl group), and there are generally known chain carbonate compounds such as dimethyl carbonate, diethyl carbonate or the like and cyclic carbonate compounds such as ethylene carbonate, propylene carbonate or the like. These carbonate compounds are used as a special solvent in the fields of medicinal chemistry, agricultural chemistry, etc.; starting or intermediate material for dyes, plant protective agents, synthetic resins, etc.; agricultural chemical or drag (see Japanese Laid-open Patent Application Nos. Sho 54-125617 and Sho 54-63023).
Of these, cyclic carbonate compounds such as ethylene carbonate and propylene carbonate have excellent characteristics as a solvent in that they dissolve well various organic and inorganic substances, are chemically and physically stable and have a high dielectric constant. Therefore, they are of industrially high utility value, and it is known that they are used in not only organic solvents but also pharmaceuticals, acrylic fiber processing agents, polymer compound solvents, organic intermediate materials, electrolyte solutions for non-aqueous batteries, electrolyte solutions for capacitors and solvents for an electrochemical reaction (see Japanese Laid-open Patent Application Nos. Sho 61-64082 and Hei 1-292753).
Some halogen-substituted cyclic carbonate compounds are known as cyclic carbonate derivatives. As chlorine-substituted cyclic carbonates, there are known, for example, chloroethylene carbonate (see J. Org. Chem., 39, 38 (1974) and the specification of U.S. Pat. No. 367795), 2,3-dichlorobutylene carbonate (see Chem. Pharm. Bull., 36, 394 (1988) and Japanese Laid-open Patent Application No. Hei 2-111767), chloromethyl ethylene carbonate (see the specification of U.S. Pat. No. 4,332,729) and trichloromethyl ethylene carbonate (see Chem. Pharm. Bull., 23, 3017 (1975)). The properties as a solvent of these chlorine atom-substituted cyclic carbonates are not known.
Meanwhile, fluorine-containing carbonate compounds are not so common and such compounds as difluoroethyl carbonate as a starting material for synthetic resins (see the specification of U.S. Pat. No. 969,683); dihexafluoropropyl carbonate and ethylhexafluoropropyl carbonate as agricultural chemicals (see the specification of U.S. Pat. No. 3,359,296); and diperfluorophenyl carbonate (see the specification of U.S. Pat. No. 768,179) and methyl-2,2,2-trifluoroethyl carbonate (see Japanese Laid-open Patent Application No. Hei 6-219992) as flame retardants have been reported as chain carbonates containing fluorine.
Known examples of fluorine atom-substituted cyclic carbonates are few and in particular, a compound obtained by introducing a fluorine atom into the methyl group of methyl ethylene carbonate is totally unknown.
As described above, although known cyclic carbonates have such excellent features as solvents that they dissolve well organic and inorganic substances, are chemically and physically stable and have a high dielectric constant, they have the following problems with properties as a solvent. For instance, though ethylene carbonate has a low molecular weight, it has a freezing temperature as high as 38.degree. C. and is solid at room temperatures. When it is used as a solvent, therefore, it must be liquefied by warming, and further, the temperature range at which it can be used as a solvent is narrow. Propylene carbonate is widely used as a non-aqueous electrolyte solvent for batteries. However, when it is used as a solvent for a lithium ion battery which uses graphite as a negative electrode material or as a solvent for a battery which uses lithium or a lithium-containing alloy as a negative electrode material, such problem is pointed out that propylene carbonate reacts with the negative electrode material to shorten the life of the battery.
Heretofore, nickel--cadmium batteries, lead batteries and the like which use an aqueous electrolytic solution have been widely used as general secondary batteries. However, new-type portable electronics such as camcorders, portable telephones, lap-top computers, etc. have been making their appearance one after another in recent years, and in this connection, a further increase in energy density is required for secondary batteries as portable power sources in order to reduce the size and weight of these electronics. Therefore, the aforesaid nickel--cadmium batteries and lead batteries are becoming unsatisfactory. Furthermore, cadmium and lead are not preferred from a viewpoint of the protection of global environment and their use has begun to be legally restricted in some countries. Under the circumstances, the development of a secondary battery which use a substitute for these materials has been desired. Attention is now paid to a non-aqueous electrolyte battery which uses a non-aqueous electrolytic solution obtained by dissolving an electrolyte in a non-aqueous solvent, as a substituent for the above nickel--cadmium batteries and lead batteries.
Since the battery in which a non-aqueous electrolytic solution is used has higher voltage and higher energy density than a battery using an aqueous electrolytic solution, it has begun to be used as a power source for electronics in civilian demands. As the non-aqueous electrolytic solution is used a mixture of a solvent having a high dielectric constant, such as propylene carbonate, .gamma.-butyrolactone or sulfolane and an electrolyte such as lithium hexafluorophosphate.
However, an electrolytic solution for these non-aqueous solution-based batteries has electric conductivity of one to two digit lower value than that of an electrolytic solution for aqueous solution-based batteries, and this is one of factors of increase in the internal resistance of a battery. Further, a non-aqueous electrolytic solution containing a non-aqueous solvent of low withstand voltage has such a defect that the charge and discharge efficiency of a battery using such solution becomes low and its service life becomes short.
To improve the electric conductivity of a non-aqueous electrolytic solution, attempts have been made to add a cyclic ether such as 1,3-dioxolanes or tetrahydrofurans or a chain ether such as 1,2-dimethoxy ethane (DME) or diethyl ether to a cyclic carbonate such as propylene carbonate (see "Denkikagaku (Electro-chemistry)", 53, No.3, 173 (1985)).
It has been reported that a carbonate having high withstand voltage, such as diethyl carbonate, is used in place of a solvent having low withstand voltage, such as dimethoxy ethane, to increase the charge and discharge efficiency of a battery as an attempt to improve the durability of an electrolytic solution (see Japanese Laid-open Patent Application No. Hei 2-10666).
It is therefore an object of the present invention to provide a novel fluorine-substituted cyclic carbonate.
It is another object of the present invention to provide a fluorine-substituted cyclic carbonate compound useful as a solvent having such excellent properties that it is chemically and physically stable, has a high dielectric constant, is capable of dissolving well organic substances and has a wide application temperature range.
It is still another object of the present invention to provide a fluorine-substituted cyclic carbonate which gives a non-aqueous electrolytic solution having excellent withstand voltage and charge and discharge cycle characteristics, a high flash point and excellent safety,
It is a further object of the present invention to provide a non-aqueous electrolytic solution containing the fluorine-substituted cyclic carbonate of the present invention and having properties as described above.
It is a still further object of the present invention to provide a non-aqueous electrolyte battery employing the non-aqueous electrolytic solution of the present invention, which is safe, can generate high voltage, and has excellent battery performance.
The above and other objects and advantages of the present invention will become apparent from the following description.